Conventional articles of athletic footwear include two primary elements, an upper and a sole structure. The upper is generally formed from a plurality of elements (e.g., textiles, foam, leather, synthetic leather) that are stitched or adhesively bonded together to form an interior void for securely and comfortably receiving a foot. The sole structure incorporates multiple layers that are conventionally referred to as a sockliner, a midsole, and an outsole. The sockliner is a thin, compressible member located within the void of the upper and adjacent to a plantar (i.e., lower) surface of the foot to enhance comfort. The midsole is secured to the upper and forms a middle layer of the sole structure that attenuates ground reaction forces during walking, running, or other ambulatory activities. The outsole forms a ground-contacting element of the footwear and is usually fashioned from a durable and wear-resistant rubber material that includes texturing to impart traction.
The primary material forming many conventional midsoles is a polymer foam, such as polyurethane or ethylvinylacetate. In some articles of footwear, the midsole may also incorporate a fluid-filled chamber that increases durability of the footwear and enhances ground reaction force attenuation of the sole structure. In some footwear configurations, the fluid-filled chamber may be at least partially encapsulated within the polymer foam, as in U.S. Pat. No. 5,755,001 to Potter, et al., U.S. Pat. No. 6,837,951 to Rapaport, and U.S. Pat. No. 7,132,032 to Tawney, et al. In other footwear configurations, the fluid-filled chamber may substantially replace the polymer foam, as in U.S. Pat. No. 7,086,180 to Dojan, et al. In general, the fluid-filled chambers are formed from an elastomeric polymer material that is sealed and pressurized, but may also be substantially unpressurized or pressurized by an external source. In some configurations, textile or foam tensile members may be located within the chamber, or reinforcing structures may be bonded to an exterior surface of the chamber to impart shape to or retain an intended shape of the chamber.
Fluid-filled chambers suitable for footwear applications may be manufactured by a two-film technique, in which two separate sheets of elastomeric film are bonded together to form a peripheral bond on the exterior of the chamber and to form a generally sealed structure. The sheets are also bonded together at predetermined interior areas to give the chamber a desired configuration. That is, interior bonds (i.e., bonds spaced inward from the peripheral bond) provide the chamber with a predetermined shape and size upon pressurization. In order to pressurize the chamber, a nozzle or needle connected to a fluid pressure source is inserted into a fill inlet formed in the chamber. Following pressurization of the chamber, the fill inlet is sealed and the nozzle is removed. A similar procedure, referred to as thermoforming, may also be utilized, in which a heated mold forms or otherwise shapes the sheets of elastomeric film during the manufacturing process.
Chambers may also be manufactured by a blowmolding technique, wherein a molten or otherwise softened elastomeric material in the shape of a tube is placed in a mold having the desired overall shape and configuration of the chamber. The mold has an opening at one location through which pressurized air is provided. The pressurized air induces the liquefied elastomeric material to conform to the shape of the inner surfaces of the mold. The elastomeric material then cools, thereby forming a chamber with the desired shape and configuration. As with the two-film technique, a nozzle or needle connected to a fluid pressure source is inserted into a fill inlet formed in the chamber in order to pressurize the chamber. Following pressurization of the chamber, the fill inlet is sealed and the nozzle is removed.